Kazusa Province was a province of Japan in the area of modern Chiba Prefecture. The province was located in the middle of the Bōsō Peninsula, whose name takes its first kanji from the name of Awa Province and its second from Kazusa and Shimōsa provinces. Its abbreviated form name was Sōshū (総州) or Nansō (南総). The borders of Kazusa Province were defined by Shimōsa Province to the north, the Pacific Ocean to the east, Awa Province to the south, and Tokyo Bay to the west.

Shimōsa Province was a province of Japan in the area of modern Chiba Prefecture and Ibaraki Prefecture as well as the bordering parts of Saitama Prefecture and Tokyo. It lies to the north of the Bōsō Peninsula (房総半島), whose name takes its first kanji from the name of Awa Province and its second from Kazusa and Shimōsa Provinces. Its abbreviated form name was Sōshū (総州) or Hokusō (北総).